DEVELOPMENT OF EXTERNAL GENITALIA IN THE HUMAN EMBRYO. 81 



whether he considers that there is a caudal migration of the labio-scrotal swellings 

 into the scrotum, or whether the final formation of the latter is accomplished by an 

 anal prolongation of these swellings with an accompanying cranial atrophy. 



Subsequent growth results in the gradual reduction of the broad urogenital 

 opening into a small aperture of varying size and shape, located just proximal to the 

 coronary sulcus. The reduction in the size of this opening is usually accompanied 

 by the development of a more or less pronounced epithelial rim and an increase in the 

 size of the epithelial tag into a comb-like ridge of varying size and shape. This 

 sequence of events is so clearly shown in figures 24 to 28, 31, and 32 that no detailed 

 description of the individual steps is necessary. Attention should be called, how- 

 ever, to the ridge-like development of the penial raphe in the embryo illustrated in 

 figure 27, where it forms a penial-scrotal frenulum, binding the penis to the scrotum 

 with a consequent decurvation. The production of this frenulum is quite character- 

 istic for most of the older embryos, although as a rule it does not become noticeably 

 developed until after the embryos have attained a length of 55 mm. 



The changes taking place in the female at this time are not so extensive. As 

 in the male, a definite coronary sulcus is formed. The other modifications may be 

 considered as a faint paralleling of the more profound metamorphosis which the 

 male genitalia have been undergoing and consist chiefly of changes in the labio- 

 scrotal swellings. The caudal ends of these draw toward each other (fig. 35) and 

 finally fuse to form the posterior commissure (figs. 37, 38), definitely separating the 

 urogenital opening from the anus, although this is slight in comparison to the 

 corresponding separation in the male. In this manner, the labio-scrotal swellings 

 are transformed from separate, lateral swellings into a cranially open, horseshoe- 

 shaped rim partially encircling the phallus. Unlike the corresponding change in 

 the male, this fusion is not accompanied by either a caudal migration of these 

 swellings or by the formation of a definite raphe". 



Regarding the formation of the posterior commissure, my observations are 

 decidedly at variance with the conclusions drawn by Wood-Jones (1913) that in the 

 adult (human) female this is not definitely present as a commissural bar separating 

 the vaginal orifice and anus, but that it merely shows as such when the genitalia are 

 placed in an unnatural position. 



The posterior commissure may be considered as marking the advent of the 

 definitive period in the female, and from now on we may refer to the component 

 parts of the external genitalia by their adult terms. The labio-scrotal swellings 

 form the labia majora, connected by the posterior commissure. The cavernous por- 

 tion of the phallus becomes the clitoris, divided into glans and shaft. The urethral 

 folds constitute the labia minora, margining the persisting primary urogenital 

 opening (the urethro-vaginal orifice) . It must be pointed out, however, that the 

 use of these latter terms at this time is an arbitrary one, because the division of the 

 female phallus into clitoris and labia minora is never entirely complete and is con- 

 summated only with the formation of the frenulum clitoridis at some time after the 

 close of the period covered by the present investigation (100 mm. CR length). 



